Experimental Studies of Aerosol-Cloud Interactions

Sara Lance

SPEC, Inc.Boulder, CO

Abstract:

Oceans cover over 70% of the Earth's surface with an albedo that is generally much less than 0.1. Yet the average albedo (the fraction of sunlight that is reflected back to space) of our planet is about 0.3. Most of this difference is due to cloud cover. Clouds are therefore clearly an important driver of the climate system. Yet there is much that is still unknown about the processes driving clouds. Of particular interest are the ways that human activities influence clouds. One of the most direct pathways involves the emission of aerosol particles that act as cloud condensation nuclei (CCN). This talk discusses past and ongoing research into aerosol-cloud interactions.

* Tools for evaluating aerosol effects on clouds. Of primary importance to the question of aerosol-cloud interactions are accurate measurements of cloud droplet concentrations. A methodology is shown for calibrating cloud droplet probes. This work demonstrates that there can be very large biases in the measurements as a result of more than one cloud droplet passing through the probe sample volume at the same time, which can easily be overlooked and misleading.

* Aerosol properties relevant for cloud formation. Measurements of CCN are shown to coincide with diurnal changes to the composition and mixing-state of aerosol particles in a megacity environment. These diurnal changes are linked with emission of small nonhygroscopic particles from fresh motor vehicle exhaust and with new particle formation events that occur when gas-phase precursors react in the presence of sunlight to create many small (<4nm) hygroscopic particles.

* Aerosol properties relevant for cloud phase changes. Observations in Arctic mixed-phase (liquid/ice) clouds are highlighted to illustrate the effect that aerosol particles can have on cloud phase transformations. CCN concentrations are postulated to affect the ice microphysical properties through modification of the droplet size distribution. Large droplets (>30 microns) have long been shown to be correlated with ice crystal concentrations. This correlation is also seen in the Arctic mixed-phase clouds presented, and is further correlated with reductions in carbon monoxide, dust particles, black carbon (soot) particles and CCN, indicating that the less polluted airmasses are more likely to contain higher concentration of ice particles. Several mechanisms are postulated for this observation, but require a better understanding of the ice nucleation mechanisms acting at these warm temperatures (> -15oC).

* New tool for studying ice nucleation mechanisms. A novel instrument for studying the mechanisms of contact nucleation using optical levitation is described. A single droplet is trapped in a laser beam and then gently bombarded by individual aerosol particles until nucleation is observed.